The present invention relates to ultrasound imaging, and more particularly, to a synthetic aperture focusing method for ultrasound imaging based on planar waves.
An ultrasound imaging system for medical use provides real-time two-dimensional (2-D) images by using transducers that transmit ultrasound signals into the human body and applying various signal processes on the reflected signals. As shown in FIG. 1, the resolution of the ultrasound image is determined by resolutions in: axial direction 2, the direction of the transmitted beam orthogonal to the width of the transducer; lateral direction 4, the direction orthogonal to axial direction 2 and in the plane of the width of the transducer; and elevational direction 6, the direction orthogonal to axial direction 2 and the width of the transducer. The axial resolution is not a critical factor in determination of the entire resolution of an ultrasound image since the axial resolution is generally three to five times higher than the other resolution components. However, unlike the axial resolution, the lateral and elevational resolutions vary depending on the transmit/receive focusing technique.
It is known that the lateral resolution can be improved by performing real-time receive dynamic focusing on all imaging points. Details of the real-time receive dynamic focusing will now be illustrated with reference FIG. 2. Referring to FIG. 2, transmission signals (not shown) of the ultrasound imaging system are delayed by the delay time corresponding to the focusing depth of each transducer 9 of transducer array 13 by transmit focusing delay 8. The transmission signals with the time delay are stored in transmission pattern memory 10 and transmit-focused to the target object(not shown) by transducer array 13 through transmitter 11 and transmitting/receiving switch 12. All transmitted beams are focused to fixed transmit focal point 14. After that, echo signals are reflected from focal point 14 and converted to electronic signals through each transducer 9 of transducer array 13. They are stored in reception pattern memory 16 through transmitting/receiving switch 12 and receiver 15. The echo signals stored in reception pattern memory 16 have different phases which vary in accordance with the different focusing depths of each transducer 9. Thus, a variable time delay is added through receive focusing delay 17 so that the echo signals are in phase. The in-phase echo signals are combined at beamformer 18 and processed at signal processor 19, and displayed on display 21 through scan converter 20.
While only one fixed transmit focal point 14 has been discussed, receive dynamic focusing is also possible with respect to all imaging points constituting one transmit scan line from echo signals obtained by a single transmission. Receive dynamic focusing is performed by compensating for the differences in distance between the transducers and the focal point.
However, the two-way dynamic focusing is performed only at the focal point 14 because the transmit focusing illustrated in FIG. 2 is performed at the focal point 14. As a result, the lateral resolution may be degraded as the beams rapidly spread if they pass through the focal point 14.
Referring to FIG. 3, the lateral resolution is degraded, depending on the focusing depth. When the delay time for the receive focusing is compensated, echo signals received by respective transducers, n1, n2, and n3, are the combination of signals reflected from reflectors 22a and 22b on curved surfaces W1, W2, and W3. Curves W1, W2, and W3 lie on circles centered on transducers n1, n2, and n3 with radii equal to the distance between each transducer and focusing depth Z1, respectively. If two reflectors 22a and 22b exist on Z1 and echo signals reflected from the two reflectors are combined, then only the power of echo signals reflected from reflector 22a on line L0 is strengthened. However, if echo signals reflected from two reflectors 24a and 24b on Z2 are combined, then the powers of echo signals reflected from reflector 24a on line L1 and reflector 24b on line L0 are simultaneously strengthened. Difference in the lateral resolution based on focusing depth arises because whereas the three curves at Z1 only overlap with respect to reflector 22a on line L0, the curves at Z2 overlap with respect to reflector 24a on line L1 and reflector 24b on line L0, lowering the lateral resolution.
It is, therefore, the objective of the present invention to provide a synthetic aperture focusing method which is capable of two-way dynamic focusing through the use of planar waves, thereby improving the lateral resolution.
In accordance with a preferred embodiment of the present invention, there is provided to a synthetic aperture focusing method for an ultrasound imaging system comprising the steps of: producing planar waves by a plurality of transducers; transmitting the planar waves to a target object; receiving signals reflected from the target object through a plurality of receive subapertures, wherein each receive subaperture is comprised of at least one of the plurality of transducers; dynamic-focusing the received signals; and combining the dynamic-focused signals to form at least one beam pattern.